Long-distance optical data transmission at bit rates on the order of 10 Gbits/sec or more often utilize optical data streams configured in a return-to-zero (RZ) format. For example, soliton data transmission applications require the generation of optical pulses that have a hyperbolic secant pulse shape or another predetermined pulse shape. In soliton transmission, the optical pulse shape is selected such that effects of wavelength-dependent negative group velocity dispersion experienced by the pulse as it travels along an optical fiber are exactly offset by effects of intensity-dependent self-phase modulation attributable to the Kerr nonlinearity. This balancing of group velocity dispersion and self-phase modulation allows the optical pulse to propagate over very long lengths of fiber while maintaining its original pulse shape. Generation of optical pulses for soliton transmission and other high bit rate applications typically requires a high-quality optical data transmitter. The optical data transmitter should be capable of generating an optical data stream in which there is no "pedestal" between adjacent pulses, and in which the pulses are substantially transform limited.
One type of conventional optical data transmitter for generating high-quality RZ optical data streams includes a laser or other optical source which generates a periodic optical pulse train at the required bit rate, and a chirp-free amplitude modulator which encodes a stream of data onto the optical pulse train. Such a transmitter is described in greater detail in L. F. Mollenauer, P. V. Mamyshev and M. J. Neubelt, "Measurement of timing jitter in filter-guided soliton transmission at 10 Gbits/sec and achievement of 375 Gbits/s-Mm, error-free, at 12.5 and 15 Gbits/sec," Optics Letters, Vol. 19, No. 10, pp. 704-706, May 1994, which is incorporated by reference herein. In another type of conventional RZ optical data transmitter, a continuous wave (CW) laser drives an amplitude modulator which also receives an RZ electrical data stream as an input modulation signal. The amplitude modulator utilizes the RZ electrical data stream to provide both pulse shaping and data encoding for the CW laser output. This type of transmitter is described in greater detail in N. M. Froberg et al., "Integrated data encoding of a 5-Gbit/sec soliton pulse train using a laser/modulator transmitter," Technical Digest of the Optical Fiber Communication Conference, 1995 OSA Technical Digest Series, Vol. 8, paper Tu15, pp. 40-41, OSA, Washington D.C., 1995, which is incorporated by reference herein. A significant problem with these and other RZ optical data transmitters which rely on the use of an amplitude modulator for pulse shaping and/or data encoding functions relates to the bias setting of the amplitude modulator. The bias setting has a tendency to drift over time, and the transmitter may therefore be unable to maintain the desired pulse shape and other characteristics for the resulting output RZ optical pulses. This undesirable variation in the transmitter output pulses as a function of time can severely undermine performance in soliton transmission systems and other high bit rate optical data transmission systems.
It is therefore apparent that a need exists for a technique for generating high-quality, high bit rate RZ optical pulses without the need for a separate amplitude modulator, and thus without the modulator bias problems associated with the above-described conventional techniques.